Hi mewla, try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

thank you

See also #answer-738461 updating the TBM addon prevents this issue ~J99

Having run in safe mode with the three addons disabled i eliminated it to TABMIX PLUS, specifically I had " use firefox session restore feature" ticked in the session tab unselecting this restores X close function

hope this helps others

I had this problem after upgrading to 38.0.1, so reverted to the previous 37.0 release. Just now I updated to 38.0.5 and the problem is still here.

Apparently it is caused by an incompatibility with the Tab Mix Plus extension. I had TabMixPlus 0.4.1.7.1

the problem is fixed if you update TMP to the latest development build which is 0.4.1.8.pre150317a1 you have to get directly it from the "Latest development build" link at the top right of their support page: http://tmp.garyr.net/support/viewtopic.php?t=2

thanks Edrandall, all sorted